This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
A fuel cell is an effective device for converting chemical energy to electrical energy through electro-catalytic reactions. The proton exchange membrane fuel cell (PEMFC) operates at a relatively low temperature with the gas phase hydrogen as fuel and oxygen (air) as oxidant. Because of its relatively high conversion efficiency, low noise and low emissions, the PEMFC is deemed to have substantial potential for use in a variety of applications, including automobiles and distributed power generation. At the core of a PEMFC is the membrane electrode assembly (MEA) which includes an anode, a cathode, and a polymer electrolyte layer in between. At the surface of the anode, hydrogen is oxidized to protons through the electro-catalytic processH2→2H++2e−  (1)
The protons thus produced are transported to the cathode side through the proton conductive polymer electrolyte layer. At the surface of the cathode, oxygen is electro-catalytically reduced and subsequently reacts with protons from equation (1) to form water:O2+4e−+4H+→2H2O  (2)
Reaction (2) is also known as the oxygen reduction reaction (ORR). Reactions (1) and (2) occur on the surface of the electrode catalysts. At present, generally the most effective catalyst for electrocatalytic reactions utilizes a platinum (Pt) electrode catalyst supported on an amorphous carbon substrate. A typical Pt loading on the MEA surface ranges from about 0.2 mg/cm2 to about 0.4 mg/cm2. Because platinum is a precious metal with limited supply, its use as a catalyst adds a significant cost to a PEMFC system. Other platinum group metals (PGMs), such as Pd, Rh, Ru, etc., are being evaluated as a possible replacement for Pt. However, PGMs also generally suffer from high cost and limited reserves. As such, the use of PGMs in electrochemical devices such as a fuel cell typically adds significant cost to the system and represents a major barrier to commercialization.
Various attempts have been made to replace PGMs in fuel cells. These attempts have been mainly focused on developing replacement materials utilizing transition metal compounds. For example, it is known that molecules containing a macrocyclic structure with an iron or cobalt ion coordinated by nitrogen from the four surrounding pyrrolic rings has catalytic activity toward capture and reduction of molecular oxygen. Additionally, ORR catalytic activity can be improved for systems containing coordinated FeN4 and CoN4 macrocycles through heat treatment. Examples of a macro-molecular system containing FeN4 and CoN4 moieties include corresponding transitional metal phthalocyanine and porphyrin.
Methods of preparing non-PGM catalyst by incorporating a transition metal into heteroatomic polymers in a polymer/carbon composite are also known. Additionally, good ORR activity can be achieved by mixing amorphous carbon based catalyst with FeN4 group and carbonaceous material or synthetic carbon support, followed by high temperature treatment in a gas mixture of ammonia, hydrogen and argon. An iron salt adsorbed on carbon in the presence of a nitrogen precursor can also produce a catalyst with good ORR activity. However, such catalyst material will generally decompose under acidic conditions to release iron, and thus is unstable for the electro-catalytic reaction within a fuel cell cathode. Additionally, because carbon does not carry the electrocatalytic activity by itself, using a carbon support dilutes the catalytic active site and results in inhomogeneous active site distribution in the final catalyst materials thus prepared.
Furthermore, other new methods of preparing the electrode catalysts for the ORR have been disclosed, such as those containing mainly transition metals, carbon and nitrogen, but free of PGMs. Such a method may include multiple steps such as (1) the synthesis of metal-organic framework (MOF) materials containing transition metals and organic ligands with or without nitrogen-containing functional groups through solvothermal reaction in solvent, (2) optionally adding another one or more transition metals into the porous structure of the MOF materials through addition in solvent, (3) optionally adding other nitrogen containing compounds into the MOF through solvent exchange, (4) Separate prepared MOF from solvent and heat-treating the MOF materials at the elevated temperatures under inert gas atmosphere, (5) optionally further heat-treat the prepared MOFs at the elevated temperature in the presence of ammonia or other N-containing chemicals, (6) optionally further treat the prepared materials with acids to remove excess metals, (7) optionally further treat the prepared material under inert gas atmosphere at elevated temperature. For a complete description, see U.S. Pub. No. 2012-0077667, which is herein incorporated by reference in its entirety.
However, although MOF materials have been demonstrated to be an effective precursor for preparing a non-PGM catalyst, the synthesis process is often costly. The MOF synthesis generally requires solvothermal reaction in solution phase for an extended period of time. Such a use of solvent for synthesis, together with the subsequent separation, adds to both the material and processing costs of MOF preparation.
In light of these considerations, there is a need to develop a low cost synthesis route to prepare non-PGM catalysts with improved catalytic activity in ORR.